Stabilization of prepare voltage of transmission storage target

ABSTRACT

A method and apparatus for improved operation of a transmission storage tube are described in which the preparation voltage provided on the storage dielectric prior to writing is stabilized to a substantial uniform value slightly positive of the flood gun cathode voltage. This is achieved by applying a plurality of stabilization pulses between the storage target electrode and the flood gun cathode during the bombardment of the storage dielectric by flood electrons, such pulses having a peak voltage greater than the first crossover voltage so the secondary emission ratio is greater than unity and having a quiescent voltage less than such first crossover voltage. In one embodiment, the frequency of the stabilization pulses is increased for a brief period at the beginning of the preparation period. The first target of a charge transfer tube is prepared for writing a charge image thereon by the stabilization pulses of the present invention which increases the quality of the charge image transferred to the second target.

llnited States Patent janko STABILIZATION OF PREPARE VOLTAGE OF TRANSMISSION STORAGE TARGET [75] Inventor: Bozidar Janko, Portland, Oreg.

[73] Assignee: Tektronix, Inc., Beaverton, Oreg.

[22] Filed: Jan. 24, 1972 [21] Appl. No.: 220,060

[52] US. Cl 328/123, 315/8.5, 315/12, 340/173 CR [51] Int. Cl. ..Gl1c 11/26, l-lOlj 29/41 [58] Field of Search 328/123, 124; 1515/85, 12; 340/173 CR [56] References Cited UNITED STATES PATENTS 3,165,665 1/1965 Finnin et al. 315/12 3,432,717 3/1969 Yaggy 315/12 2,903,618 9/1959 Smith 315/12 2,953,711 9/1960 Taubenslag 315/12 3,088,048 4/1963 Ogland et al. 315/12 3,089,056 5/1963 Lehrer 315/12 Primary Examiner-Stanley D. Miller, Jr.

Attorney-Stephen W. Blore, James Campbell, Jr.

et al.

A method and apparatus for improved operation of a transmission storage tube are described in which the preparation voltage provided on the storage dielectric prior to writing is stabilized to a substantial uniform value slightly positive of the flood gun cathode voltage. This is achieved by applying a plurality of stabilization pulses between the storage target electrode and the flood gun cathode during the bombardment of the storage dielectric by flood electrons, such pulses having a peak voltage greater than the first crossover voltage so the secondary emission ratio is greater than unity and having a quiescent voltage less than such first crossover voltage. In one embodiment, the frequency of the stabilization pulses is increased for a brief period at the beginning of the preparation period. The first target of a charge transfer tube is prepared for writing a charge image thereon by the stabilization pulses of the present invention which increases the quality of the charge image transferred to the second target.

ABSTRACT 10 Claims, 5 Drawing Figures STABILIZATION OF PREPARE VOLTAGE OF TRANSMISSION STORAGE TARGET BACKGROUND OF THE INVENTION The subject matter of the present invention relates generally to charge image storage tubes having transmission mesh storage targets, and in particular to an improved method and apparatus for operating such tubes to stabilize the preparation voltage provided on the storage target dielectric prior to writing a charge image thereon. This voltage stabilization is achieved by applying a plurality of pulses to the target electrode having a peak voltage above and a quiescent voltage below the first crossover voltage of the storage dielectric to cause flood electrons to charge such dielectric alternately positively and negatively until it reaches a stabilized prepare voltage which was slightly positive with respect to the flood gun cathode. As a result, flood electrons continue to bombard the storage dielectric and maintain the prepare voltage at a substantially uniform value over the surface of the dielectric.

The present invention is especially useful when employed to operate a charge transfer type of transmission storage tube in which a charge image is written on a first storage target of fast writing speed and is later transferred to a second target capable of long storage time. The preparation voltage provided on the first storage target dielectric is stabilized to a substantially uniform value prior to writing a charge image thereon and this greatly increases the quality of the charge image transferred to the second target.

Charge transfer storage tubes, such as that disclosed in copending US Pat. application, Ser. No. 180,420 filed on Sept. 14, 1971, now US. Pat. No. 3,710,179, STORAGE TUBE HAVING TRANSMISSION TAR- GET WITH LOW DIFFERENTIAL CUTOFF" by R. Hayes and W. Hayward have difficulty in transferring good quality charge images to the second target due to residual charge images remaining on ,the first target after erasure of such target, as well as to preparation voltage variations on the first target storage dielectric before writing occurs caused by target nonuniformities and uncontrolled shifts in voltage due to charge redistribution with the dielectric as well as positive ion bombardment of the dielectric. These problems are solved to a great extent by the prepare voltage stabilization pulsing technique of the present invention.

It is, therefore, one object of the present invention to provide an improved operation for charge image storage apparatus having a transmission storage target.

Another object of the present invention is to provide such an improved operation in which the prepare voltage provided on the storage target dielectric prior to writing is stabilized to a substantially uniform value.

A further object of the present invention is to provide such an improved operation in which the prepare voltage is stablized by applying a plurality of stabilization pulses between the target mesh electrode and the flood gun cathode so that the flood electrons bombard the dielectric at energies greater than the first crossover voltage of such dielectric during the pulses and at energies below such first crossover voltage between such pulses to charge the storage dielectric alternately positively and negatively until it reaches a stabilized prepare voltage slightly positive with respect to the flood gun cathode.

A still further object of the present invention is to provide such a storage tube apparatus and method of operation in which the frequency of the stabilization pulses is greater at the start of the preparation period in order to more quickly charge the storage target dielectric to the stabilized prepare voltage.

Still another object of the invention is to provide such an improved apparatus and method for stabilizing the prepare voltage on the first storage target of a charge transfer storage tube before the charge image is written on the first target and subsequently transferred to a second target.

An additional object of the present invention is to provide such an improved charge transfer storage tube apparatus and method of operation in which the storage dielectric of the first target is prepared for writing by charging such dielectric to a stabilized prepare voltage which is positive with respect to the flood gun cathode in order to enable charge images of better quality to be transferred to the second target.

BRIEF DESCRIPTION OF DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of certain preferred embodiments thereof and from the attached drawings of which:

FIG. 1 is a side elevation view of a charge transfer storage tube apparatus in accordance with one embodiment of the present invention with a portion of the tube envelope broken away for clarity;

FIG. 2 is a schematic diagram of electrical voltage waveforms which are applied to the two storage targets and collector electrode in the tube of FIG. 1;

FIGS. 3A and 33, respectively, show the prior art prepare voltages and the stabilized prepare voltages of the present invention provided on the first storage target in FIG. 1; and

FIG. 4 shows the voltage waveform applied to the first storage target in the tube of FIG. 1 in accordance with another embodiment of the operation of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS As shown in FIG. 1, one embodiment of a charge image storage tube apparatus operated in accordance with the present invention includes a charge transfer storage tube 10 of the type shown in copending US. Pat. application, Ser. No. 180,420 of R. Hayes and W. Hayward filed Sept. 14, 1971, entitled Storage Tube Having Transmission Target with Low Differential Cutoff." The charge transfer storage tube 10 includes a first transmission mesh storage target 12, a second transmission mesh storage target 14 and a separate phosphor viewing screen 16. The phosphor viewing screen 16 is supported on the inner surface of a glass face plate 18 sealed to a hollow funnel 20 which may be made of ceramic material to form the evacuated envelope of the tube. The phosphor screen includes a layer 22 of phosphor material coated on the inner surface of the faceplate 18 at one end of the envelope and an acceleration electrode 24 formed by a layer of aluminum or other conductive material coated over the surface of the phosphor screen and connected to an external source of positive high voltage DC. potential around +5 kilovolts.

A writing gun 26 is supported within the other end of the envelope and includes a writing gun cathode 28 connected to an external source of negative high voltage D.C. potential of about 3 kilovolts. The writing gun includes the usual control grid 30 and focusing anodes 32 which focus the high velocity writing electrons emitted by cathode 28 into a narrow electron beam 34. The writing beam 34 is transmitted between a pair of horizontal deflection plates 36 and a pair of vertical deflection plates 38 which deflect such beam horizontally and vertically in accordance with electrical signal voltages applied thereto in a conventional manner to cause the writing beam to form a charge image on a storage dielectric layer 40 of the first storage target 12.

The storage dielectric layer 40 of the first transmission storage target I2 is provided on the left side of a first mesh target electrode 42 facing the writing gun in such a manner that the mesh apertures are left open. In order to provide this first target with an extremely fast writing speed, the storage dielectric layer 40 may be made of highly porous insulating material such as magnesium oxide having a density of about percent or less of its maximum bulk density and having a thickness on the order of to microns. The target electrode 42 may be an electro-formed nickel mesh of about 250 lines per inch. A pair of flood guns 44 each having grounded cathodes 46 and focusing anodes 47 are provided between the output of the vertical deflection plates 38 and a first collimating electrode 48. Low velocity flood electrons emitted from the flood gun cathodes 46 are transmitted as two broad defocused electron beams 50 which bombard the storage dielectric of the first target substantially uniformly. Some of the flood electrons are transmitted through such first target to the second target 14 and to the phosphor screen 16 in order to transfer the charge image from the first target to the second target and to produce a light image corresponding to such charge image in the manner hereafter described.

The low velocity flood electrons are transmitted through the first collimating electrode 48 as well as through second and third collimating electrodes 52 and 54 which may be in the form of wall bands of silver or other conducting material coated on the inner surface of the envelope funnel 20 and insulatingly spaced from each other. These collimating electrodes 48, 52 and 54 are connected to different D.C. potentials of, for example, volts, volts and volts, respectively, in order to spread the flood electrons uniformly over the surface of the storage target and cause such flood electrons to strike the storage dielectric 40 substantially perpendicular thereto. A collector electrode mesh 56 is provided between the third collimating electrode 54 and the first target 12 in order to collect secondary electrons emitted by storage dielectric 40 of such first target, and to collect the secondary electrons emitted from the storage dielectric of the second target 14 after the charge image has been transferred to such second target.

As shown in FIG. 2, a collector voltage signal 58 which changes in steps between about and +150 volts is applied by a first pulse generator to a terminal 60 in FIG. 1 connected to the collector electrode 56. A first target voltage signal 62 is applied by a second pulse generator to another terminal 64 connected to the first target electrode 42. A second target voltage signal 66 is applied by a third pulse generator to a third terminal 68 connected to a second target mesh electrode 70 of the second target 14. The three pulse generators are all formed of conventional oscillator and pulse shaper circuits and for this reason will not be described in detail.

The second storage target 14 is capable of longer storage time but is of slower writing speed than the first target 12. Any suitable secondary emissive insulator capable of bistable storage of a charge image for an indefinite time may be employed as a storage dielectric layer 72 on the left side of the second target mesh electrode 70. For example, it has been found that a thin, dense layer of magnesium oxide formed by conventional vapor deposition technique or any other suitable manner is satisfactory for the second storage dielectric 72. Thus, while the first storage dielectric 40 and the second storage dielectric 72 are both made of magensium oxide, the first dielectric is of much lower density and greater thickness so that the first target has a lower capacitance and, therefore, a faster writing speed than the second target. However, the second target has a much longer storage time than the first target and is capable of providing bistable storage while the first target is operated as a halftone storage target for maximum writing speed. A charge image is initially written on the first storage target 12 and then subsequently transferred to the second storage target 14 by the transmission of flood electrons through the written or charge image areas of the first target. This is accomplished by employing the voltage signals 58, 62 and 66 of FIG. 2. In other words, the charge transfer storage tube 10 combines the high writing speed of a half-tone storage target and the long storage time of a bistable storage target.

As shown in FIG. 2, during the Prepare T2 time period the second storage target 14 is erased and prepared for writing. To do this the first target electrode voltage 62 is held at about volts to enable the low velocity flood electrons to uniformly pass through such first target and strike the second target. During this time, the second target electrode voltage 66 rapidly increases from a quiescent value of +1 50 volts to a maximum of about +300 volts which is greater than the first crossover voltage of the second target dielectric 72 so that the flood electrons cause the entire dielectric to uniformly fade positive to a corresponding voltage and thereby erase any charge image previously stored thereon. Next, the second target voltage 66 is rapidly decreased in value from +300 volts to zero volts so that the flood electrons uniformly charge the second storage dielectric negatively to the flood gun cathode voltage of zero volts. The second target electrode voltage 66 is then gradually increased from zero volts to an operating level of volts at a rate sufficiently slow so that there is no appreciable capacitive coupling between such target electrode and the second storage dielectric, thereby enabling the potential of such storage dielectric to be maintained at zero volts by the flood electrons. During this time, the collector voltage 58 is decreased from +150 volts to +105 volts and is returned at the end of the period to +128 volts. The second target is now prepared for writing a charge image thereon.

During the Prepare T1 time period the first storage target 12 is prepared for writing by decreasing the first target voltage 62 to about +20 volts which is less than the first crossover voltage of about +60 volts of the first storage dielectric 40. The secondary emission characteristic curve 74 of the first storage dielectric 40 has a first crossover voltage point 76 of about +60 volts where it first crosses the unity secondary emission ratio line of such dielectric. At target voltages below the first crossover voltage 76, the secondary emission ratio of the dielectric is less than one so that the low velocity flood electrons charge the dielectric negatively down to a first stable voltage 78 approximately equal to the zero volts potential of the flood gun cathode. However, for target voltages above the first crossover voltage 76, the low velocity flood electrons charge the storage dielectric positively towards a second stable voltage 80, ap proximately equal to the collector electrode potential.

With prior art methods of preparation of the first storage target 12, the voltage 62' applied to the first target electrode 42 was maintained constant at volts during the entire Prepare Tl period, as shown by voltage waveform 82 in FIG. 3A. As a result, the voltage on the surface of the first dielectric 40 is charged negatively to the first stable state voltage 78 slightly negative with respect to the zero voltage potential of the flood gun cathode as shown by dashed line voltage curve 84 in FIG. 3A. At the end of the preparation period, a charge image is written on the first target dielectric 40 by the writing beam 34 at Write time 86. Immediately after the charge image is written on the first target, its target electrode voltage is decreased by a voltage A V from the 20 volts level 82 to a lower value 88 to enable such charge image to be transferred to the second storage target 14.

As shown in FIG. 2, this charge transfer is accomplished by increasing the second target electrode voltage 66 from +150 volts to a high voltage of about +800 volts, thereby enabling the flood electrons transmitted through the positive written areas of the first target to be accelerated sufficiently to strike the second target with a high enough velocity to write a corresponding charge image on the second target dielectric 72 with a potential above the first crossover voltage of the second target. It should be noted that the reason the first target electrode potential 62 is decreased by A V to level 88 during charge transfer is the high field produced by the second target electrode voltage 66 when it is increased to +800 volts which would otherwise draw the flood electrons through the unwritten areas of the first target. As a result, the transferred charge image is stored bistably on such second target for an indefinite time by uniform bombardment of the second dielectric 72 with the flood electrons during the"B.S. Store period. This bistable storage on the second target is accomplished by increasing the first target electrode voltage 62 from the level 88 of about +18 volts to a higher voltage level 90 of about +120 volts which enables the flood electrons to be uniformly transmitted through said first target and to uniformly bombard the second target to bistably store the transferred charge image on such second target and to display it on the phosphor screen. At the same time, the collector voltage 58 is increased in value from +128 volts to +150 volts.

As shown in H6. 3A, during the prior art operation of the charge transfer storage tube, the surface potential 84 of the first storage dielectric 40 decreases to a negative value below the zero volt potential of the flood gun cathode so that the flood electrons are repelled and prevented from striking the first storage dielectric. As a result, the dielectric potential 84 varies in an unstable, uncontrolled manner due to target nonuniformities, positive ion'bombardment, internal charging, etc., since the flood electrons can no longer strike the dielectric to maintain it at a stable potential. When the target electrode voltage 62 is decreased A V from potential 82 to potential 88, the dielectric surface potential 84 is decreased by a corresponding amount A V to a lower level 92 of uncertain value. Since the dielectric potential 92 of the unwritten areas of the first target dielectric 40 varies and is of an uncertain value, as indicated by arrow 93, charge images of poor quality are transferred to the second target when the charge image potential 94 of the written areas of the first dielectric is only a few tenths of a volt greater than the potential 92 of the unwritten background areas. Thus, using the prior art operation, low voltage images cannot be tranferred to the second target without also transferring the potential on the unwritten background areas of the first target which results in erasure of a portion of the low voltage images on the second target.

In order to avoid the above problem, the present invention uses a plurality of positive stabilization pulses 96 which are applied to the first target electrode 42 during the preparation period PREPARE T1, as shown in FIG. 2 by a conventional pulse generator which is turned on at the end of the negative erase pulse in signal 66 and turned off when writing occurs at time 86. The stabilization pulses 96 are rectangular pulses which may have an amplitude of about volts and are added to the quiescent voltage level 82 of +20 volts of the first target electrode during this time so that such pulses reach a maximum voltage of volts which is greater than the first crossover voltage 76 of the first storage dielectric 40. As shown in FIG. 3B, the stabilization pulses 96 cause the surface potential 84 of the first storage dielectric 40 to momentarily increase in value due to capacitive coupling to a voltage greater than the first crossover voltage 76. As a result, the flood electrons cause the first dielectric to charge positively at potential portions 98 during such pulses. The net overall effect of the positive charging 98 during the stabilization pulses 96 and the negative charging of potential portions 100 between such pulses is that an equilibrium prepare voltage 102 is reached by the first storage dielectric immediately prior to writing which is positive with respect to the zero volts potential of the flood gun cathode, so that flood electrons continue to strike the storage dielectric to stabilize such prepare voltage. As a result, the stabilized prepare voltage 102 is maintained at substantially the same value at the time of writing 86 regardless of when writing occurs. This means that when the potential of the first target electrode is decreased by an amount A V to the voltage level 88 suitable for transfer of the charge image, the potential of the unwritten portions of the first dielectric decreases by an amount A V to a voltage level 92' which is also substantially constant. Therefore, charge image potentials 94 of extremely low value may be transferred to the second storage target without also transferring the potential 92' of the unwritten target areas. While the amplitude, pulse width, and frequency of the stabilization pulses 96 can vary, it has been found that pulses about 2 microseconds wide and having a frequency of 100 hertz or a period of 10- milliseconds are satisfactory. This has been found to provide a stabilized prepare voltage 102 on the order of about +0.1 volt with respect to the grounded flood gun cathode.

As shown in FIG. 4, another embodiment of the present invention includes a plurality of high frequency stabilization pulses 104 which are provided at the start of the Prepare Tl" preparation period for the first storage target. The high frequency stabilization pulses are of a greater frequency than the other stabilization pulses 96 applied to the first target during the remainder of the period and may be on the order of about 100 times the frequency of pulses 96 when they are of the same width and amplitude. This causes the potential of the first storage dielectric 40 to more rapidly reach the stabilized prepare voltage 102, and thereby enables writing to take place earlier. It should be noted that while repetitive rectangular pulses may be used for the stabilization pulses 96, any suitable waveform pulses can be employed as long as their amplitude exceeds the first crossover voltage 76.

It will be obvious to those having ordinary skill in the art that many changes may be made in the details of the above-described preferred embodiments of the present invention. For example, the method and apparatus of the present invention may be employed on conventional single target transmission storage tubes in order to improve the quality of the light image produced on the phosphor viewing screen by providing better contrast with respect to the unwritten background areas. Therefore, the scope of the present invention should only be determined by the following claims.

I claim: 1. A method of operation transmission storage tube having a transmission mesh storage target including a mesh target electrode and a storage dielectric layer provided thereon, in which the improvement comprises:

bombarding the storage dielectric layer substantially uniformly with low velocity flood electrons to charge the bombarded surface of said dielectric to a prepare voltage which prepares the target for writing thereon; applying a pulse train including a plurality of successive stabilization pulses between the target electrode and the flood electron cathode during the preparation bombardment of said dielectric by said flood electrons, said pulses each having a maximum voltage greater than the first crossover voltage of the secondary emission characteristic of said dielectric so its secondary ratio is greater than unity for the flood electrons, the quiescent voltage of said pulses being less than said first crossover voltage, and said pulses causing said dielectric to charge positively during the pulses and to charge negatively between said pulses until said prepare voltage is stabilized to a value which is positive with respect to the voltage of said flood electron cathode and below said first crossover voltage; and

thereafter bombarding the stabilized storage dielectrio with high velocity writing electrons having energies above said first crossover voltage when said prepare voltage is stabilized, to write a positive charge image thereon and thereby enabling a greater amount of flood electrons to be transmitted through the written areas of said target.

2. A method in accordance with claim 1 in which the stabilization pulses are repetitive pulses of substantially rectangular waveform.

3. A method in accordance with claim 2 in which the stabilization pulses have a greater frequency at the start of preparation of the target than the frequency during the remainder of such preparation.

4. A method in accordance with claim 1 in which the storage tube also includes a second storage target and the flood electrons transmitted through the written areas of the first mentioned target are caused to bombard the storage dielectric of said second target in order to transfer the charge image from the first target to the second target.

5. A method in accordance with claim 4 in which the second storage target is also a transmission mesh target and the flood electrons are transmitted through the written charge image areas of the second target to a separate phosphor screen which emits a light image corresponding to the charge image.

6. A transmission storage tube apparatus in which the improvement comprises:

a transmission mesh storage target including a mesh target electrode and a storage dielectric layer provided on said target electrode leaving open the mesh apertures;

flood means including a cathode for bombarding said storage dielectric layer substantially uniformly with low velocity flood electrons to charge the bombarded surface of said dielectric to a prepare voltage which prepares the target for writing thereon;

pulse application means for applying a pulse train including a plurality of successive stabilization pulses between said target electrode and said flood electron cathode during the preparation bombardment of said dielectric by said flood electrons, said pulses each having a maximum voltage sufficient to make the secondary emission ratio of said dielectric greater than unity for the flood electrons, the quiescent voltage of said pulses being a smaller voltage where said ratio is less than unity, and said pulses having a pulse width and spacing such that said prepare voltage is stabilized at a substantially constant value which is positive with respect to the cutoff voltage of said target and where said ratio is less than unity thereby enabling the flood electrons to continue to strike said dielectric to maintain said prepare voltage substantially uniformly over the surface of said dielectric layer;

a collector electrode for collecting the secondary electrons emitted by said dielectric; and

writing means including a cathode for bombarding the stabilized storage dielectric with high velocity writing electrons at voltages where the secondary emission ratio of the dielectric is greater than unity, to write a charge image on said dielectric layer when said prepare voltage is stabilized.

7. An apparatus in accordance with claim 6 in which the pulse application means applies repetitive stabilization pulses of rectangular waveform to the target electrode.

8. An apparatus in accordance with claim 7 in which said pulses are of a greater frequency at the start of the preparation of the target than at the end of such preparation.

9. An apparatus in accordance with claim 6 in which the storage tube also includes a storage target and a transfer means for causing flood electrons to be transmitted through the written areas of the first mentioned target to bombard the storage dielectric of said second target in order to transfer the charge image from said first target to said second target.

10. An apparatus in accordance with claim 9 in which the second storage target is also a transmission mesh target and the flood electrons are transmitted through the written charge image areas of the second target to a separate phosphor screen which emits a light image corresponding to the charge image.

19! t '0 i i UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,753 1-29 Dated Auqust l4 1973 'Inventofls) BOZIDAR JANKO It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 45, after "tribution" "with" should be within-;

Column 6, lines 15 and 16, "tranferred" should be -transferred-w Column 7 line 30 after "operation" should be -of Signed and sealed this 8th day of January 1974.

(SEAL). v A

Attest: V a

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Attest ng Officer Acting Commissioner of Patents 5 FORM PC4050 (19-69) v USCOMM-DC scan-p09 U45. GOVERNHENY PRINTING OFFICE: I! OII6-IJ| UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,753,129 Dated August 14 1973 Inventor(s) BOZIDAR JANKO It: is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 45, after "tribution" "with" should be -within;

Column 6, lines 15 and 16, "tranferred" should be -transferred-; I

Column 7, line 30 after "operation" should be --of Signed and sealed this 8th day of January 1974.

(SEAL) v Arrest: i

EDWARD M.FLETCHER,JR. RENE D. TEGTMEYER Attesting Officer Acting Commissioner of Patents 

1. A method of operation transmission storage tube having a transmission mesh storage target including a mesh target electrode and a storage dielectric layer provided thereon, in which the improvement comprises: bombarding the storage dielectric layer substantially uniformly with low velocity flood electrons to charge the bombarded surface of said dielectric to a prepare voltage which prepares the target for writing thereon; applying a pulse train including a plurality of successive stabilization pulses between the target electrode and the flood electron cathode during the preparation bombardment of said dielectric by said flood electrons, said pulses each having a maximum voltage greater than the first crossover voltage of the secondary emission characteristic of said dielectric so its secondary ratio is greater than unity for the flood electrons, the quiescent voltage of said pulses being less than said first crossover voltage, and said pulses causing said dielectric to charge positively during the pulses and to charge negatively between said pulses until said prepare voltage is stabilized to a value which is positive with respect to the voltage of said flood electron cathode and below said first crossover voltage; and thereafter bombarding the stabilized storage dielectric with high velocity writing electrons having energies above said first crossover voltage when said prepare voltage is stabilized, to write a positive charge image thereon and thereby enabling a greater amount of flood electrons to be transmitted through the written areas of said target.
 2. A method in accordance with claim 1 in which the stabilization pulses are repetitive pulses of substantially rectangular waveform.
 3. A method in accordance with claim 2 in which the stabilization pulses have a greater frequency at the start of preparation of the target than the frequency during the remainder of such preparation.
 4. A method in accordance with claim 1 in which the storage tube also includes a second storage target and the flood electrons transmitted through the written areas of the first mentioned target are caused to bombard the storage dielectric of said second target in order to transfer the charge image from the first target to the second target.
 5. A method in accordance with claim 4 in which the second storage target is also a transmission mesh target and the flood electrons are transmitted through the written charge image areas of the second target to a separate phosphor screen which emits a light image corresponding to the charge image.
 6. A transmission storage tube apparatus in which the improvement comprises: a transmission mesh storage target including a mesh target electrode and a storage dielectric layer provided on said target electrode leaving open the mesh apertures; flood means including a cathode for bombarding said storage dielectric layer substantially uniformly with low velocity flood electrons to charge the bombarded surface of said dielectric to a prepare voltage which prepares the target for writing thereon; pulse application means for applying a pulse train including a plurality of successive stabilization pulses between said target electrode and said flood electron cathode during the preparation bombardment of said dielectric by said flood electrons, said pulses each having a maximum voltage sufficient To make the secondary emission ratio of said dielectric greater than unity for the flood electrons, the quiescent voltage of said pulses being a smaller voltage where said ratio is less than unity, and said pulses having a pulse width and spacing such that said prepare voltage is stabilized at a substantially constant value which is positive with respect to the cutoff voltage of said target and where said ratio is less than unity thereby enabling the flood electrons to continue to strike said dielectric to maintain said prepare voltage substantially uniformly over the surface of said dielectric layer; a collector electrode for collecting the secondary electrons emitted by said dielectric; and writing means including a cathode for bombarding the stabilized storage dielectric with high velocity writing electrons at voltages where the secondary emission ratio of the dielectric is greater than unity, to write a charge image on said dielectric layer when said prepare voltage is stabilized.
 7. An apparatus in accordance with claim 6 in which the pulse application means applies repetitive stabilization pulses of rectangular waveform to the target electrode.
 8. An apparatus in accordance with claim 7 in which said pulses are of a greater frequency at the start of the preparation of the target than at the end of such preparation.
 9. An apparatus in accordance with claim 6 in which the storage tube also includes a storage target and a transfer means for causing flood electrons to be transmitted through the written areas of the first mentioned target to bombard the storage dielectric of said second target in order to transfer the charge image from said first target to said second target.
 10. An apparatus in accordance with claim 9 in which the second storage target is also a transmission mesh target and the flood electrons are transmitted through the written charge image areas of the second target to a separate phosphor screen which emits a light image corresponding to the charge image. 